It has been widely practiced, for the purpose of preventing corrosion, to coat the internal surface and external surface of a metallic can with a material obtained by dissolving or dispersing in a solvent a thermosetting resin of various types, such as epoxy-based resin, phenol-based resin or the like, and thereby cover the metal surfaces. However, such a method of covering with a thermosetting resin has unfavorable problems such as the time consuming process of drying the coating material, a reduction in productivity, environmental pollution due to a large amount of the organic solvent, and the like.
As a method for solving these problems, there is a method that laminates a film on a material of a metallic can, that is, a steel sheet or an aluminum sheet, or one of these metallic sheets that has been surface-treated in a manner such as plating or the like. If a film-laminated metallic sheet is drawn or ironed to produce metallic cans, the film is required to have the following properties:
(1) The film should be excellent in adhesion to a metallic sheet. PA1 (2) The film should be excellent in formability and does not cause a defect such as a pinhole after the forming process. PA1 (3) The film should not be peeled, cracked or pinholed by impact on the metallic can. PA1 (4) The polyester film should not adsorb a flavor component of the content of the metallic can or should cause degradation of the flavor of the content of the metallic by a substance dissolved from the film (hereinafter, referred to as "taste property"). PA1 .sigma.: relative standard deviation PA1 D: number average particle diameter (.mu.m) PA1 Di: particle diameter (.mu.m) PA1 n: number of particles (number) PA1 Si: number of moles of silicon atoms in 100 g of particles, PA1 Al: number of moles of aluminum atoms in 100 g of particles, PA1 M: number of moles of alkaline metal atoms in 100 g of particles.
Many proposals have been made to meet these requirements. For example, Japanese patent application laid-open No. Sho 64-22530 discloses a polyester film having specific density and face orientation coefficient; Japanese patent application laid-open No. Hei 2-57339 discloses a copolymerized polyester Film having specific crystallinity; and Japanese patent application laid-open No. Hei No. 6-218895, Japanese patent application laid-open No. Hei 6-107815, etc., disclose a polyester film coplymerized with an isophthalic acid component containing specific particles, or the like. However, these proposals cannot satisfy all the various requirements as stated above, and cannot be said to be at sufficiently satisfying levels especially in applications where both a high level formability and an excellent taste property after retorting are required.
An object of the present invention is to solve the aforementioned problems of the conventional art and to provide a biaxially stretched polyester film for forming containers that is excellent in formability, heat resistance and taste property and, more particularly, that is suitable for metallic cans produced by a forming process and having an excellent taste property.